Boreal Scots Pine Research Articles

Role of de novo biosynthesis in ecosystem scale monoterpene emissions from a boreal Scots pine forest

Abstract. Monoterpene emissions from Scots pine have traditionally been assumed to originate as evaporation from specialized storage pools. More recently, the significance of de novo emissions, originating directly from monoterpene biosynthesis, has been recognized. To study the role of biosynthesis at the ecosystem scale, we measured monoterpene emissions from a Scots pine dominated forest in southern Finland using the disjunct eddy covariance method combined with proton transfer reaction mass spectrometry. The interpretation of the measurements was based on a correlation analysis and a hybrid emission algorithm describing both de novo and pool emissions. During the measurement period May–August 2007, the monthly medians of daytime emissions were 200, 290, 180, and 200 μg m−2 h−1. The emissions were partly light dependent, probably due to de novo biosynthesis. The emission potential for both de novo and pool emissions exhibited a decreasing summertime trend. The ratio of the de novo emission potential to the total emission potential varied between 30 % and 46 %. Although the monthly changes were not significant, the ratio always differed statistically from zero, suggesting that the role of de novo biosynthesis was observable. Given the uncertainties in this study, we conclude that more accurate estimates of the contribution of de novo emissions are required for improving monoterpene emission algorithms for Scots pine dominated forests.

Modeling seasonal course of carbon fluxes and evapotranspiration in response to low temperature and moisture in a boreal Scots pine ecosystem

Environmental conditions act above and below ground, and regulate carbon fluxes and evapotranspiration. The productivity of boreal forest ecosystems is strongly governed by low temperature and moisture conditions, but the understanding of various feedbacks between vegetation and environmental conditions is still unclear. In order to quantify the seasonal responses of vegetation to environmental factors, the seasonality of carbon and heat fluxes and the corresponding responses for temperature and moisture in air and soil were simulated by merging a process-based model (CoupModel) with detailed measurements representing various components of a forest ecosystem in Hyytiälä, southern Finland. The uncertainties in parameters, model assumptions, and measurements were identified by generalized likelihood uncertainty estimation ( GLUE). Seasonal and diurnal courses of sensible and latent heat fluxes and net ecosystem exchange ( NEE) of CO 2 were successfully simulated for two contrasting years. Moreover, systematic increases in efficiency of photosynthesis, water uptake, and decomposition occurred from spring to summer, demonstrating the strong coupling between processes. Evapotranspiration and NEE flux both showed a strong response to soil temperature conditions via different direct and indirect ecosystem mechanisms. The rate of photosynthesis was strongly correlated with the corresponding water uptake response and the light use efficiency. With the present data and model assumptions, it was not possible to precisely distinguish the various regulating ecosystem mechanisms. Our approach proved robust for modeling the seasonal course of carbon fluxes and evapotranspiration by combining different independent measurements. It will be highly interesting to continue using long-term series data and to make additional tests of optional stomatal conductance models in order to improve our understanding of the boreal forest ecosystem in response to climate variability and environmental conditions.

Climate, canopy conductance and leaf area development controls on evapotranspiration in a boreal coniferous forest over a 10-year period: A united model assessment

The aim of this work was to test a process-based model (hydrological model combined with forest growth model) on the simulation of seasonal variability of evapotranspiration (ET) in an even-aged boreal Scots pine ( Pinus sylvestris L.) stand over a 10 year period (1999–2008). The water flux components (including canopy transpiration ( E t) and evaporation from canopy ( E c) and ground surface ( E g) were estimated in order to output the long-term stand water budget considering the interaction between climate variations and stand development. For validation, half-hourly data on eddy water vapor fluxes were measured during the 10 growing seasons (May–September). The model predicted well the seasonal course of ET compared to the measured values, but slightly underestimated the water fluxes both in non-drought and drought (2000, 2003 and 2006) years. The prediction accuracy was, on average, higher in drought years. The simulated ET over the 10 years explained, on average, 58% of the daily variations and 84% of the monthly amount of ET. Water amount from E t contributed most to the ET, with the fractions of E t, E c and E g being, on average, 67, 11 and 23% over the 10-year period, respectively. Regardless of weather conditions, the daily ET was strongly dependent on air temperature ( T a) and vapor pressure deficit ( D a), but less dependent on soil moisture ( W s). On cloudy and rainy days, there was a non-linear relationship between the ET and solar radiation ( R o). During drought years, the model predicted lower daily canopy stomatal conductance ( g cs) compared with non-drought years, leading to a lower level of E t. The modeled daily g cs responded well to D a and W s. In the model simulation, the annual LAI increased by 35% between 1999 and 2008. The ratio of E c: ET correlated strongly with LAI. Furthermore, LAI reduced the proportion of E g as a result of the increased share of E c and E t and radiation interception. Although the increase of LAI affected positively E t, the contribution of E t in ET was not significantly correlated with LAI. To conclude, although the model predicted reasonably well the seasonal course of ET, the calculation time steps of different processes in the model should be homogenized in the future to increase the prediction accuracy.

Boreal pine forest floor biogenic volatile organic compound emissions peak in early summer and autumn

In boreal forests, canopy-scale emissions of biogenic volatile organic compounds (BVOCs) are rather well characterised, but knowledge of ecosystem-scale BVOC emissions is still inadequate. We used adsorbent tubes to measure BVOCs from a boreal Scots pine ( Pinus sylvestris L.) forest floor in southern Finland and analysed the compounds with a gas chromatograph–mass spectrometer. The most abundant compound group was the monoterpenes (averaging 5.04 μg m −2 h −1), in which α-pinene, Δ 3-carene and camphene contributed over 90% of the emissions. Emissions of other terpenoids (isoprene and sesquiterpenes) were low (averaging 0.05 and 0.04 μg m −2 h −1, respectively). BVOC emissions from the forest floor varied seasonally, peaking in early summer and autumn, with most of the compounds following similar patterns. The emission pattern was sustained throughout the measurement period, suggesting that the main sources of the emissions remained more or less stable. We compared the BVOC fluxes with environmental parameters such as temperature, precipitation and PAR, and with fluxes of other trace gases (CO 2, CH 4, N 2O), as well as with ground vegetation photosynthesis and with litter input. Several of these parameters were correlated with the presence of BVOCs. The sources of soil BVOC emissions are very poorly understood, but our results suggest, that changes in litter quantity and quality, soil microbial activity and the physiological stages of plants are linked with changes in BVOC fluxes.

Photosynthesis of ground vegetation in boreal Scots pine forests

Research on carbon uptake in boreal forests has mainly focused on mature trees, even though ground vegetation species are effective assimilators and can substantially contribute to the CO2 uptake of forests. Here, I examine the photosynthesis of the most common species of ground vegetation in a series of differently aged Scots pine stands, and at two clear-cut sites with substantial differences in fertility. In general, the biomass of evergreen species was highest at poor sites and below canopies, whereas grasses and herbs predominated at fertile open sites. Unlike mosses, the measured vascular species showed clear annual cycles in their photosynthetic activity, which increased earlier and decreased later in evergreen vascular species than in deciduous species. However, intraspecific variation and self-shading create differences in the overall level of photosynthesis. Light, temperature history, soil moisture and recent possible frosts could explain the changes in photosynthesis of low shrubs and partially also some changes in deciduous species. Light and the occurrence of rain events explained most of the variation in the photosynthesis of mosses. The photosynthetic production of ground vegetation was first upscaled, using species-specific and mass-based photosynthetic activities and average biomass of the site, and then integrated over the growing season, using changes in environmental factors. Leaf mass-based photosynthesis was highest in deciduous species, resulting in notably higher photosynthetic production at fertile sites than at poor clear-cut sites. The photosynthetic production decreased with stand age, because flora changed towards evergreen species, and light levels diminished below the canopy. In addition, the leaf mass-based photosynthetic activity of some low shrubs declined with the age of the surrounding trees. Different measuring methods led to different momentary rate of photosynthesis. Therefore, the choice of measuring method needs special attention.

Tree influence on carbon stock and C:N ratio of soil organic layer in boreal Scots pine forests

Observed small-scale spatial variation of forest soil is suggested to be produced by tree influence. We examined spatial variation of the tree influences by modelling tree influence potential that accounts for location and size of trees. Thereafter, we tested significance of the correlations between the tree influence potential and soil properties (carbon stock and C:N ratio of the organic layer) with Monte Carlo permutation tests. The methods were applied to five Scots pine stands located in the boreal vegetation zone in Finland. We found statistically significant tree influence on soil C:N ratio in all studied stands, but the tree influence on soil carbon stock was significant only in three of five stands. This indicates that location of trees has a relatively steady and remarkable influence on spatial variation of a soil parameter that reflects soil fertility and nutrient balance, but variation of soil carbon stock is not that clearly affected by current tree stand. The correlations between C:N ratios and tree influences were positive in four of the five stands, indicating that soil was less fertile near the trees. The methods described here produce statistically reliable information pertaining to the influence of trees on soil properties.

Seasonal and inter-annual variability of energy exchange above a boreal Scots pine forest

Abstract. Twelve-years of eddy-covariance measurements conducted above a boreal Scots pine forest in Hyytiälä, Southern Finland, were analyzed to assess the seasonal and inter-annual variability of surface conductance (gs) and energy partitioning. The gs had distinct annual course, driven by the seasonal cycle of the Scots pine. Low gs (2–3 mm s−1 in April) cause the sensible heat flux to peak in May–June while evapotranspiration takes over later in July–August when gs is typically 5–7 mm s−1. Hence, during normal years Bowen ratio decreases from 4–6 in April to 0.7–0.9 in August. Sensitivity of gs to ambient vapor pressure deficit (D) was relatively constant but the reference value at D = 1 kPa varied seasonally and between years. Only two drought episodes when volumetric soil moisture content in upper mineral soil decreased below 0.15 m3 m−3 occurred during the period. Below this threshold value, transpiration was strongly reduced, which promoted sensible heat exchange increasing Bowen ratio to 3–4. Annual evapotranspiration varied between 218 and 361 mm and accounted between 50% and 90% of equilibrium evaporation. The forest floor contributed between 16 and 25% of the total evapotranspiration on annual scale. The fraction stayed similar over the observed range of environmental conditions including drought periods. The inter-annual variability of evapotranspiration could not be linked to any mean climate variable while the summertime sensible heat flux and net radiation were well explained by global radiation. The energy balance closure varied annually between 0.66 and 0.95 and had a distinct seasonal cycle with worse closure in spring when a large proportion of available energy is partitioned into sensible heat.

Determination of de novo and pool emissions of terpenes from four common boreal/alpine trees by 13CO2 labelling and PTR‐MS analysis

Boreal forests emit a large amount of monoterpenes into the atmosphere. Traditionally these emissions are assumed to originate as evaporation from large storage pools. Thus, their diurnal cycle would depend mostly on temperature. However, there is indication that a significant part of the monoterpene emission would originate directly from de novo synthesis. By applying 13CO2 fumigation and analyzing the isotope fractions with proton transfer reaction mass spectrometry (PTR-MS) and classical GC-MS, we determined the fractions of monoterpene emissions originating from de novo biosynthesis in Pinus sylvestris (58%), Picea abies (33.5%), Larix decidua (9.8%) and Betula pendula (100%). Application of the observed split between de novo and pool emissions from P. sylvestris in a hybrid emission algorithm resulted in a better description of ecosystem scale monoterpene emissions from a boreal Scots pine forest stand.

Autumn temperature and carbon balance of a boreal Scots pine forest in Southern Finland

Abstract. We analyzed the dynamics of carbon balance components: gross primary production (GPP) and total ecosystem respiration (TER), of a boreal Scots pine forest in Southern Finland. The main focus is on investigations of environmental drivers of GPP and TER and how they affect the inter-annual variation in the carbon balance in autumn (September–December). We used standard climate data and CO2 exchange measurements collected by the eddy covariance (EC) technique over 11 years. EC data revealed that increasing autumn temperature significantly enhances TER: the temperature sensitivity was 9.5 gC m−2 °C−1 for the period September–October (early autumn when high radiation levels still occur) and 3.8 gC m−2 °C−1 for November–December (late autumn with suppressed radiation level). The cumulative GPP was practically independent of the temperature in early autumn. In late autumn, air temperature could explain part of the variation in GPP but the temperature sensitivity was very weak, less than 1 gC m−2 °C−1. Two models, a stand photosynthesis model (COCA) and a global vegetation model (ORCHIDEE), were used for estimating stand GPP and its sensitivity to the temperature. The ORCHIDEE model was tested against the observations of GPP derived from EC data. The stand photosynthesis model COCA predicted that under a predescribed 3–6 °C temperature increase, the temperature sensitivity of 4–5 gC m−2 °C−1 in GPP may appear in early autumn. The analysis by the ORCHIDEE model revealed the model sensitivity to the temporal treatment of meteorological forcing. The model predictions were similar to observed ones when the site level 1/2-hourly time step was applied, but the results calculated by using daily meteorological forcing, interpolated to 1/2-hourly time step, were biased. This is due to the nonlinear relationship between the processes and the environmental factors.

Carbon balance and component CO2 fluxes in boreal Scots pine stands

This study quantifies and analyses the dynamics of carbon balance and component carbon dioxide (CO2) fluxes in four Southern Finnish Scots pine stands that covered the typical economic rotation time of 80 years. The study was based on direct flux measurements with chambers and eddy covariance (EC), and modelling of component CO2 fluxes. The annual CO2 balance varied from a source of about 400 g C m a at a recently clearcut site to net CO2 uptake of 200–300 g C m a in a middle-aged and a mature stand. A 12-year-old sapling site was at the turning point from source to a sink of CO2. In the middle-aged stand, photosynthetic production was dominated by trees. Under closed pine canopies, ground vegetation accounted for 10–20% of stand photosynthesis whereas at the open sites the proportion and also the absolute photosynthesis of ground vegetation was much higher. The aboveground respiration was dominated by tree foliage which accounted for one third of the ecosystem respiration. Rate of wood respiration was in the order of 10% of total ecosystem respiration. CO2 efflux from the soil dominated the ecosystem respiratory fluxes in all phases of stand development. Instantaneous and delayed responses to the environmental driving factors could predict well within-year variability in photosynthetic production: In the short term and during the growing season photosynthesis follows primarily light while the seasonal variation is more strongly connected to temperature. The temperature relationship of the annual cycle of photosynthesis was found to be almost equal in the southern boreal zone and at the timberline in the northern boreal zone. The respiratory fluxes showed instantaneous and seasonal temperature relationships but they could also be connected to photosynthesis at an annual timescale.

Underestimation of Terpene Exposure in the Nordic Wood Industry

This study determined that emission of sesquiterpenes from processed wood warrants attention in the work environment. Currently, only the monoterpenes in the terpene group are monitored in occupational hygiene studies. Terpene emissions are a work environment issue for industries that process wood, as they are known to cause respiratory difficulties and mucous membrane irritation. Fresh sawdust of the most common boreal conifers, Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), was subjected to processing (drying), and the emissions were analyzed with a gas chromatograph-mass spectrometer. The data indicate that workers are exposed to significant amounts of sesquiterpenes, an observation that has not been recorded previously at wood processing plants. On average, the proportion of sesquiterpenes to monoterpenes was 21 ± 5% (STD, n = 11) for spruce and 15 ± 5% (STD, n = 13) for pine. The composition of terpenes emitted in air from spruce wood differs from the composition in resin. The sum of monoterpenes and sesquiterpenes can exceed the occupational exposure limit for turpentine for processes where monoterpene concentrations are already close to the occupational exposure limit, and for processes involving the processing of bark. Findings suggest that future studies of health effects from terpenes in air should measure monoterpenes and sesquiterpenes to assess whether the current OELs are appropriate.

Decomposition of Scots pine fine woody debris in boreal conditions: Implications for estimating carbon pools and fluxes

Litter quality and environmental effects on Scots pine ( Pinus sylvestris L.) fine woody debris (FWD) decomposition were examined in three forestry-drained peatlands representing different site types along a climatic gradient from the north boreal (Northern Finland) to south (Southern Finland) and hemiboreal (Central Estonia) conditions. Decomposition (percent mass loss) of FWD with diameter ≤10 mm (twigs) and FWD with diameter >10 mm (branches) was measured using the litter bag method over 1–4-year periods. Overall, decomposition rates increased from north to south, the rate constants ( k values) varying from 0.128 to 0.188 year −1 and from 0.066 to 0.127 year −1 for twigs and branches, respectively. On average, twigs had lost 34%, 19% and 19%, and branches 25%, 17% and 11% of their initial mass after 2 years of decomposition at the hemiboreal, south boreal and north boreal sites, respectively. After 4 years at the south boreal site the values were 48% for twigs and 42% for branches. Based on earlier studies, we suggest that the decomposition rates that we determined may be used for estimating Scots pine FWD decomposition in the boreal zone, also in upland forests. Explanatory models accounted for 50.4% and 71.2% of the total variation in FWD decomposition rates when the first two and all years were considered, respectively. The variables most related to FWD decomposition included the initial ash, water extractives and Klason lignin content of litter, and cumulative site precipitation minus potential evapotranspiration. Simulations of inputs and decomposition of Scots pine FWD and needle litter in south boreal conditions over a 60-year period showed that 72 g m −2 of organic matter from FWD vs. 365 g m −2 from needles accumulated in the forest floor. The annual inputs varied from 5.7 to 15.6 g m −2 and from 92 to 152 g m −2 for FWD and needles, respectively. Each thinning caused an increase in FWD inputs, up to 510 g m −2, while the needle inputs did not change dramatically. Because the annual FWD inputs were lowered following the thinnings, the overall effect of thinnings on C accumulation from FWD was slightly negative. The contribution of FWD to soil C accumulation, relative to needle litter, seems to be rather minor in boreal Scots pine forests.

Predicting the decline in daily maximum transpiration rate of two pine stands during drought based on constant minimum leaf water potential and plant hydraulic conductance

The effect of drought on forest water use is often estimated with models, but comprehensive models require many parameters, and simple models may not be sufficiently flexible. Many tree species, Pinus species in particular, have been shown to maintain a constant minimum leaf water potential above the critical threshold for xylem embolism during drought. In such cases, prediction of the relative decline in daily maximum transpiration rate with decreasing soil water content is relatively straightforward. We constructed a soil-plant water flow model assuming constant plant conductance and daily minimum leaf water potential, but variable conductance from soil to root. We tested this model against independent data from two sites: automatic shoot chamber data and sap flow measurements from a boreal Scots pine (Pinus sylvestris L.) stand; and sap flow measurements from a maritime pine (Pinus pinaster Ait.) stand. To focus on soil limitations to water uptake, we expressed daily maximum transpiration rate relative to the rate that would be obtained in wet soil with similar environmental variables. The comparison was successful, although the maritime pine stand showed carry-over effects of the drought that we could not explain. For the boreal Scots pine stand, daily maximum transpiration was best predicted by water content of soil deeper than 5 cm. A sensitivity analysis revealed that model predictions were relatively insensitive to the minimum leaf water potential, which can be accounted for by the importance of soil resistance of drying soil. We conclude that a model with constant plant conductance and minimum leaf water potential can accurately predict the decline in daily maximum transpiration rate during drought for these two pine stands, and that including further detail about plant compartments would add little predictive power, except in predicting recovery from severe drought.

Impacts of climate change and forest management on monoterpene emission and needle secondary compounds of Boreal Scots pine (Pinus sylvestris L.)

Impacts of climate change and forest management on monoterpene emission and needle secondary compounds of Boreal Scots pine (Pinus sylvestris L.)

Parametrization of two photosynthesis models at the canopy scale in a northern boreal Scots pine forest

Parametrization of two photosynthesis models at the canopy scale in a northern boreal Scots pine forest

Climate change impact on snow and soil temperature in boreal Scots pine stands

Scenarios indicate that the air temperature will increase in high latitude regions in coming decades, causing the snow covered period to shorten, the growing season to lengthen and soil temperatures to change during the winter, spring and early summer. To evaluate how a warmer climate is likely to alter the snow cover and soil temperature in Scots pine stands of varying ages in northern Sweden, climate scenarios from the Swedish regional climate modelling programme SWECLIM were used to drive a Soil-Vegetation-Atmosphere Transfer (SVAT)-model (COUP). Using the two CO2 emission scenarios A and B in the Hadley centres global climate model, HadleyA and HadleyB, SWECLIM predicts that the annual mean air temperature and precipitation will increase at most 4.8°C and 315 mm, respectively, within a century in the study region. The results of this analysis indicate that a warmer climate will shorten the period of persistent snow pack by 73–93 days, increase the average soil temperature by 0.9–1.5°C at 10 cm depth, advance soil warming by 15–19 days in spring and cause more soil freeze–thaw cycles by 31–38%. The results also predict that the large current variations in snow cover due to variations in tree interception and topography will be enhanced in the coming century, resulting in increased spatial variability in soil temperatures.

Parametrization of two photosynthesis models at the canopy scale in a northern boreal Scots pine forest

In thiswork, eddy covariance data from a northern boreal Scots pine (Pinus sylvestris) stand were used in the parametrization of two up-scaled leaf-level photosynthesis models. The parametrization was carried out by eddy covariance data inversion. The biochemically based Farquhar model parameters, the maximum rate of electron transport (Jmax) and maximum rate of carboxylation [Vc(max)], were both obtained from the temperature responses measured by the eddy covariance. The semi-empirical model, based on optimizing water use and carbon gain, was parametrized according to the seasonal behaviour of the parameter β The parametrization of the models was performed for the year 2001, while 2002 was used as a test year to study the models’ capabilities. Both of the models tracked daily CO2 assimilation fairly well, reaching the high growing-season rates in mid-June and starting the autumn drawdown at the beginning of September. The incapability of the models to track the diminishing of CO2 fluxes during very dry days brings up the issue of combining a soil model into the canopy model. The biochemical model parameters have temperature responses that change during the growing season. The biochemical model responds more to the temperature, whereas the semi-empirical model is strongly driven by the light level.

Modelling five years of weather-driven variation of GPP in a boreal forest

This study uses a five-year series of half-hourly eddy covariance (EC) and soil temperature measurements to estimate the time development of Gross Primary Production (GPP) of a boreal Scots pine ( Pinus sylvestris L.) forest, and compares this estimate with results from three versions of a mechanistic canopy photosynthesis model at different temporal scales. The mechanistic model is independent of the canopy-level measurements, scaling up instantaneous shoot-level photosynthesis in spatially distributed crowns. It consists of submodels for radiation interception and shoot photosynthesis. The interception model calculates the attenuation of photosynthetically active radiation (PAR) through randomly distributed individual crowns. To assess the significance of driving variables at different time scales, we used three shoot photosynthesis models with an increasing number of driving variables, starting from PAR only, adding air humidity and stomatal conductance, and a delayed temperature effect describing the temperature acclimation of photosynthetic capacity. The scaling model based on incoming PAR only explained up to 95% of the within-day variation in the EC-based estimate of GPP. The model accounting for PAR and the temperature acclimation explained about 90% of the day-to-day variation in the EC-based estimate of GPP over the five years. Models without the temperature acclimation did not capture the day-to-day variation during spring. From mid to late summer, PAR was sufficient to explain the observed day-to-day variation also. Including vapour pressure deficit (VPD) was not significant for the quantitative accuracy of the results in this boreal Scots pine stand. The mechanistic models also explained most of the year-to-year variation in the EC-based estimates, except that they all yielded larger predictions in 1999, most likely due to soil drought which was not accounted for by the models.

Impacts of long-term elevation of atmospheric carbon dioxide concentration and temperature on the establishment, growth and mortality of boreal Scots pine branches

The aim of this study was to investigate the impacts of elevated carbon dioxide (CO2) and temperature on the establishment, growth and mortality of the branches of Scots pine (Pinus sylvestris L). In 1997, 16 young trees were individually enclosed in chambers, in eastern Finland, for a period of 5 years (1997–2001), in an environment that simulated the future climate for the region. There were four replicates of each treatment, including combinations of ambient and elevated CO2 and temperature. Measurements were carried out on the establishment of new branches, branch diameter growth and branch mortality. Elevated temperature and elevated CO2 had no positive effect on the number of branches that established each year or branch diameter growth. They were, instead, related to tree height growth and stem diameter growth, respectively. However, elevated CO2 and temperature caused an increase in branch mortality.

Regeneration patterns in boreal Scots pine glades linked to cold-induced photoinhibition

Regeneration patterns of Pinus sylvestris L. juveniles in central Siberian glades were studied in relation to cold-induced photoinhibition. Spatial distribution of seedlings in different height classes revealed higher seedling densities beneath the canopy than beyond the canopy, and significantly higher densities of seedlings < 50 cm tall on the north side of the trees. These patterns coincided with differences in light conditions. Compared with plants on the north side of canopy trees (north-exposed), photosynthetic photon flux (PPF) received by plants on the south side of canopy trees (south-exposed) was always higher, making south-exposed plants more susceptible to photoinhibition, especially on cool mornings. Chlorophyll fluorescence data revealed lower photochemical efficiency and increased non-photochemical quenching of small (20-50 cm in height), south-exposed seedlings from spring to early autumn, indicating increased excitation pressure on photosynthesis. Maximum rate of oxygen evolution was less in south-exposed plants than in north-exposed plants. Increased pools of xanthophyll cycle pigments and formation of the photoprotective zeaxanthin provided further evidence for the higher susceptibility to photoinhibition of south-exposed seedlings. A linear mixed model analysis explained many of the physiological differences observed in seedlings according to height class and aspect with early morning temperature and PPF as predictors. The link between photoinhibition and differential distribution of seedlings by height class suggests that photoinhibition, together with other environmental stresses, decreases the survival of small, south-exposed P. sylvestris seedlings, thereby significantly affecting the regeneration pattern of central Siberian pine glades.